Lithography in semiconductor processing relates generally to the process of transferring patterns which correspond to desired circuit components onto one or more thin films which overlie a substrate. One important step within the field of lithography involves optical tools and methods for transferring the patterns to the films which overlie the semiconductor wafer. Patterns are transferred to a film by imaging various circuit patterns onto a photoresist layer which overlies the film on the wafer. This imaging process is often referred to as "exposing" the photoresist layer. The benefit of the exposure process and subsequent processing allows for the generation of the desired patterns onto the film on the semiconductor wafer, as illustrated in prior art FIGS. 1a-1f.
Prior art FIG. 1a illustrates a photoresist layer 10 deposited by, for example, spin-coating, on a thin film 11 such as, for example, silicon dioxide (SiO.sub.2) which overlies a substrate 12 such as silicon. The photoresist layer 10 is then selectively exposed to radiation 13 (e.g., ultraviolet (UV) light) via a photomask 14 (hereinafter referred to as a "mask") to generate one or more exposed regions 16 in the photoresist layer 10, as illustrated in prior art FIG. 1b. Depending on the type of photoresist material utilized for the photoresist layer 10, the exposed regions 16 become soluble or insoluble in a specific solvent which is subsequently applied across the wafer (this solvent is often referred to as a developer).
When the exposed regions 16 are made soluble, a positive image of the mask 14 is produced in the photoresist layer 10, as illustrated in prior art FIG. 1c, and the photoresist material is therefore referred to as a "positive photoresist". The exposed underlying areas 18 in the film 11 may then be subjected to further processing (e.g., etching) using the photoresist layer 10 as a hard mask to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1d (wherein the photoresist layer 10 has been removed). Conversely, when the exposed regions 16 are mode insoluble, a negative image of the mask 14 is produced in the photoresist 10 layer, as illustrated in prior art FIG. 1e, and the photoresist material is therefore referred to as a "negative photoresist". In a similar manner, the exposed underlying areas 20 in the film 11 may then be subjected to further processing (e.g., etching) using the remaining photoresist layer 10 as a hard mask to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1f.
Photolithography systems use both refractive and reflective type optical systems. In either case, non-uniformities in one or more of the illumination system (which provides or supplies the radiation for exposure), the imaging system and the mask construction undesirably cause critical dimension non-uniformities on a target substrate or film surface. That is, two mask features which are intended to be identical exhibit differing critical dimensions at the substrate or film surface across the image field. An exemplary prior art photolithography process which illustrates such a phenomena is illustrated in prior art FIGS. 2a-2c.
Prior art FIG. 2a illustrates a fragmentary cross section of a photomask 50 having a transmissive substrate portion 51 with two patterns 52 and 54 thereon, respectively. The patterns 52 and 54 represent features to be transferred to a target substrate or film such as a silicon wafer, an oxide or a conductive layer (not shown) via radiation (e.g., ultraviolet (UV) light) 56 which passes through the mask 50. The patterns 52 and 54 absorb the radiation 56 which is incident thereon, and thus forms a radiation pattern which is incident on the target substrate or film to form the desired patterns thereon which correspond to the patterns 52 and 54.
Due to a variety of anomalies, such as illumination or exposure non-uniformities, lens imaging aberrations, or mask non-uniformities or defects, the intensity profiles of the radiation 56 at the surface of the target substrate or film corresponding to the patterns 52 and 54 are not the same, as illustrated in prior art FIG. 2b. Prior art FIG. 2b illustrates the radiation intensity profile 60 generated by the mask features 52 and 54. As is evident, a maximum radiation intensity I.sub.o is achieved at the target substrate surface or film in regions 64 where the radiation 56 was permitted to pass through the mask 50. In regions 66 and 68 corresponding to the mask patterns or features 52 and 54, parabolic-type profiles 70 and 72 exist, respectively. Note that the intensity profiles 70 and 72 are not ideal (as illustrated by a dotted line profile 74) due to the diffraction of the radiation caused by the mask features 52 and 54. In addition, and more importantly for this discussion, the profiles 70 and 72 differ from one another although the patterns 52 and 54 which generated the profiles 70 and 72 were intended to be identical (please note that the differences in profiles 70 and 72 are exaggerated in FIG. 2a for easy reference).
Prior art FIG. 2c is a fragmentary plan view of a target substrate or film 80 such as a silicon wafer, insulating layer, or conductive film having a polymethylmethacrylate (PMMA) or some other similar photoresist thereon. As evident from prior art FIG. 2b, the radiation intensity profiles 70 and 72 are not the same, thus the exposure of the radiation sensitive film 82 overlying the target substrate 80 at regions 84 and 86 differ, thus causing the critical dimensions of regions 84 and 86 to similarly differ. That is, the critical dimension of region 84 (CD.sub.1) is larger than the critical dimension of region 86 (CD.sub.2) because the intensity profile 70 is not as "tight" as the intensity profile 72 (i.e., CD.sub.1 &gt;CD.sub.2). Consequently, the various anomalies (e.g., due to illumination, imaging and/or mask non-uniformities) undesirably result in a critical dimension non-uniformity of CD.sub.1 -CD.sub.2 =.DELTA..sub.1 across the image field. Therefore there is a need in the art for a method of reducing critical dimension non-uniformities across the image field.